This invention relates to a process for the removal hydrogen peroxide from spent pickle liquor. More specifically, this invention provides a thermal process for the decomposition of hydrogen peroxide that is present in spent pickle liquor generated from a pickling process for steel.
When hydrogen peroxide is used as a component in an acidic pickle liquor solution, some amount of unreacted hydrogen peroxide remains in the spent pickle liquor waste stream. This spent pickle liquor is typically neutralized prior to its disposal. The unreacted hydrogen peroxide may be present in both the rinse water and in the spent pickle liquor solution. When considerable amounts of unreacted hydrogen peroxide are present in the spent pickle liquor, the acid neutralization step can be problematic. Thus, it is desirable to remove hydrogen peroxide from spent pickle liquor.
Conventional methods for removal of hydrogen peroxide from spent pickle liquor consist of the addition of reagents that chemically react with and consume the hydrogen peroxide present in the pickle liquor. Traditionally, the addition of sodium bisulfite, ferrous sulfate, or metallic iron to the spent pickle liquor have been used to decompose any hydrogen peroxide present in the solution and form water and oxygen. The use of sodium bisulfite is expensive. The use of ferrous sulfate or metallic iron result in an increased volume of solid sludge waste during the neutralization step.
The present invention provides an economically viable means for the removal of hydrogen peroxide from spent pickle liquor prior to acid neutralization.
The present invention provides an apparatus and a process for the removal of hydrogen peroxide from spent pickle liquor used in a steel strip pickling process.
Generally, the apparatus comprises a tank equipped with an inlet, an outlet, at least one vent, a heat source and an internal baffle. The tank is of sufficient capacity to accommodate a quantity of pickle liquor solution generated by a pickling process, wherein the pickle liquor residence time in the tank is from about 30 minutes to about 60 minutes and preferably is about 45 minutes. The heat source may be any type known to one skilled in the art, for instance, steam injection or at least one immersion heater.
Generally, the process comprises flowing the spent pickle liquor into an enclosed and vented decomposition tank through an inlet. The tank is of sufficient capacity to accommodate the quantity of solution generated by a pickling process. The spent pickle liquor is then heated to at least about 65xc2x0 C. and maintained at a temperature of at least about 65xc2x0 C. in the decomposition tank for about 30 minutes to about 60 minutes. In another embodiment, the pickle liquor is heated to at least 76xc2x0 C. Once heated for the appropriate amount of time, the heated pickle liquor is flowed out of the tank through an outlet.
Generally, the heated pickle liquor is flowed out of the decomposition tank at about the same rate as the pickle liquor is flowed into the tank to achieve a pickle liquor residence time sufficient to decompose the hydrogen peroxide in the decomposition tank, typically about 45 minutes. The decomposition tank also comprises at least one vent and an internal baffle. Any heating method known in the art may be used and typical heat sources include steam heat or at least one immersion heater. Given the above parameters for the decomposition tank, it would be obvious to one skilled in the art to vary the temperature of the tank, flow rate of the pickle liquor solution and residence time of the pickle liquor in the decomposition tank for effective decomposition of the hydrogen peroxide.